Conventionally, a trench metal oxide semiconductor-field effect transistor (MOSFET) (disclosed in, for example, Japanese Patent Application Laid-Open Publications No. 2008-218711 (Patent Document 1) and No. 2005-57050 (Patent Document 2)) is used for a switching power-supply (hereinafter, referred to as VR: voltage regulator) of supplying power to a central processor unit (CPU) in a computer or a server. Since the trench MOSFET has a smaller cell pitch than that of a planar MOSFET (disclosed in, for example, “A Novel Planar Power MOSFET With Laterally Uniform Body and Ion-Implanted JFET Region.”, IEEE Electron Device Letter, 2008, vol. 29, no. 4, pp. 375to 377, April, 2008, written by J. Ng et al., (Non-Patent Document 1)), its channel width per unit area is large, and therefore, the trench MOSFET has characteristics that ON resistance can be decreased. However, a facing area of its trench gate with its drain region is large, and therefore, the trench MOSFET has a disadvantage point of a large feedback capacity.
In recent years, the number of output capacitors of suppressing CPU-voltage variation caused when CPU consumption current is changed has been increased in order to increase a current and decrease a voltage in the CPU, which results in increasing a size and cost of the VR. It is known that improvement of a switching frequency of the VR is effective for decreasing the number of output capacitors (disclosed in, for example, “Analysis of the power delivery path from the 12-V VR to the microprocessor”, in Proc., IEEE APEC' 04, 2004, vol. 1, pp. 285 to 291, written by Y. Ren et al. (Non-Patent Document 2) or “Small signal modeling of a high bandwidth voltage regulator using coupled inductor”, IEEE Trans. Power Electron., vol. 22, no. 2, pp. 399 to 406, March 2007, written by M. Xu et al. (Non-Patent Document 3)).
A bottle neck in the improvement of the switching frequency is that a temperature of a MOSFET exceeds an upper limit of an operating temperature (for example, 150° C.) due to losses caused by the switching. As the losses caused in the switching, there are turn-on loss, turn-off loss, and drive loss for a high-side MOSFET of the VR, and conduction loss and recovery loss of an embedded diode and drive loss for a low-side MOSFET. Among the losses, the turn-on loss and the turn-off loss for the high-side MOSFET are relatively largely occupied. Hereinafter, the turn-on loss and the turn-off loss are collectively called a switching loss.
Decrease of the feedback capacity of the MOSFET is effective for decrease of the switching loss. This is because, as the feedback capacity becomes small, a switching speed becomes large, and therefore, the switching loss is decreased. There is a problem that a trench MOSFET essentially has a large feedback capacity, and therefore, the further improvement of the switching frequency is difficult.
Although a lateral-type MOSFET is cited as a structure capable of decreasing the feedback capacity, it has a disadvantage point of a large ON resistance. As reasons for the large ON resistance of the lateral-type MOSFET, the following two can be cited.
First, a region of maintaining a breakdown voltage is provided in a horizontal direction with respect to a surface of a semiconductor substrate, and therefore, when a high breakdown voltage is required, increase of a distance in the horizontal direction is required, and a cell pitch is increased, and as a result, the ON resistance is increased.
Second, drain and source electrodes are pulled from the substrate surface, and therefore, resistances of the electrodes, that is spreading resistances, are increased when a drain current is flown in the horizontal direction with respect to the substrate surface.
As means of solving the second problem, it is suggested that, the source electrode or the drain electrode is provided on a rear surface of the semiconductor substrate, so that a current is flown toward the rear surface of the substrate through a diffusion layer or a conductive substance such as a metal or polysilicon (disclosed in, for example, Japanese Patent Application Laid-Open Publications No. 2002-368121 (Patent Document 3) and No. H06-232396 (Patent Document 4)). By providing the source or drain electrode on the rear surface, areas of the source and drain electrodes are increased, and therefore, the spreading resistances of the electrodes can be decreased.